1. Field of the Invention
The present invention relates to vapor compression systems and, more particularly, to a transcritical multi-stage vapor compression system having an intermediate pressure vessel or receiver.
2. Description of the Related Art
Vapor compression systems are used in a variety of applications including heat pump, air conditioning, and refrigeration systems. Such systems typically employ refrigerants, or working fluids, that remain below their critical pressure throughout the entire vapor compression cycle. Some vapor compression systems, however, such as those employing carbon dioxide as the working fluid, typically operate as transcritical systems wherein the working fluid is compressed to a pressure exceeding its critical pressure and wherein the suction pressure of the working fluid is less than the critical pressure of the working fluid. The basic structure of such a system includes a compressor for compressing the working fluid to a pressure that exceeds its critical pressure, followed by removal of heat from the working fluid in a first heat exchanger, e.g., a gas cooler. The pressure of the working fluid discharged from the gas cooler is reduced in an expansion device and then converted to a vapor in a second heat exchanger, e.g., an evaporator, before being returned to the compressor.
FIG. 1 illustrates a typical transcritical vapor compression system 10. In the illustrated example, a two stage compressor is employed having a first compression mechanism 12 and a second compression mechanism 14. The first compression mechanism compresses the working fluid from a suction pressure to an intermediate pressure. An intercooler 16 is positioned between the first and second compression mechanisms and cools the intermediate pressure working fluid. The second compression mechanism then compresses the working fluid from the intermediate pressure to a discharge pressure that exceeds the critical pressure of the working fluid. The working fluid is then cooled in a gas cooler 18. In the illustrated example, a suction line heat exchanger 20 further cools the high pressure working fluid before the pressure of the working fluid is reduced by expansion device 22. The working fluid then enters evaporator 24 where it is boiled and cools a secondary medium, such as air, that may be used, for example, to cool a refrigerated cabinet. The working fluid discharged from the evaporator 24 passes through the suction line heat exchanger 20 where it absorbs thermal energy from the high pressure working fluid before entering the first compression mechanism 12 to repeat the cycle.
The capacity and efficiency of such a transcritical system can be regulated by regulating the pressure of the high pressure portion, e.g., the pressure in gas cooler 18, of the system. The pressure of the high side gas cooler may, in turn, be regulated by regulating the mass of working fluid contained therein which is dependent upon the total charge of working fluid actively circulating through the system.